FIG. 1 illustrates how sound waves are collected by the outer ear 1 in humans to create hearing. Acoustic waves travel down the ear canal 2, wherein frequencies are enhanced in the 4,000 cycles/second range, and then impinge on the ear drum 3 causing it to vibrate in a complex manner. The ear drum transmits this vibrational energy to the three small bones, the malleus 4, the incus 5 and the stapes 6, which transmit and amplify the sound. These are located in the air space behind the ear drum, known as the middle ear 7. The acoustic energy is further transmitted by the innermost bone, the stapes, which fits like a plunger into a window in the cochlea known as the oval window 8, which is connected to the inner ear, or cochlea 52. The vibrational energy from the stapes is thereby converted to pressure waves within the cochlea, which contains two larger and one smaller channels which are arranged in a spiral fashion of approximately two and a quarter turns, as depicted in FIG. 2, which, for simplicity, is shown uncoiled.
The external sound is thus transmitted, firstly, into one channel of the cochlea 52, the scala vestibuli 10, where it travels to the apex of the channel. At the apex the pressure wave traverses an opening known as the helicotrema 11 into a second spiral channel, the scala tympani 12, continuing along this channel inside the spiral to the round window 9. This arrangement of two spiral channels, separated by a thin membrane, activates a sensitive mechanism known as the Organ of Corti 13 illustrated in FIG. 3. This contains approximately 15,000 hair cells 14 in each ear, in a central channel between the scala vestibuli 10 and the scala tympani 12, known as the cochlear duct 15. These are best illustrated in the sketch of the cross-section of the cochlear channels of FIG. 3. These hair cells respond to the sound originating from the ear canal and act essentially as bionic transducers that change acoustic energy into electrochemical neural responses. The latter are transmitted along the auditory nerve 16 to the brain, where the neural signals are processed in specialized areas of the brain by the auditory nuclei. These have far greater ability to develop in infants--a phenomenon known as brain plasticity. This is also the case in the learning of languages. It is therefore recognized that the ideal age for treatment of deafness is as early as possible, to take advantage of the brain's ability to adapt shortly after birth.
In many cases of deafness, the hair cells or the Organ of Corti are damaged, but the auditory nerves and their cell bodies are present in sufficient numbers to process speech if they are adequately electrically stimulated. This has been clearly described in numerous publications, including the seminal work by Harold F. Schuknecht, M.D, and Mark R. Gacek, M.D., Cochlear Pathology in Presbycusis, Annals of Otology, Rhinology, and Laryngology 1993;102:1-16 Supplement 158. This study suggested that hearing could be mediated electrically. Early attempts were made to stimulate the auditory nerve electrically during neurosurgical procedures or operations in which the auditory nerve was exposed, as in the case of Djoumo and Eyries (Prosthese auditive par excitation electrique a distance du nerf sensoriel a laide d'un bobinage inclus a demeure, 1957, Presse Medicae35:14-17).
Modern developments to help the deaf include cochlear implant devices which pick up sound, process it, and deliver it in some way to the auditory nerve. Such developments have been well summarized by Clarke et al., Cochlear Prostheses edited by Graeme M. Clark; Yit. Tong & James F. Patrick, Churchill Livingstone, Edinburgh, London, Melbourne and New York, 1990. ISBN 0-443-03582-2.
Numerous inventions have been made regarding the implantation of electrodes to stimulate the auditory nerve. Chouard implanted multiple electrodes in the bony wall of the cochlea and later into the inner ear, as related in Chouard CH, McLeod P. 1976, Implantation of multiple intra-cochlear electrodes for rehabilitation of total deafness, Preliminary Report, Laryngoscope 86. 1743-1746. Similarly, Michelson and House experimented with intracochlear electrodes (William F. House, 1976. Cochlear Implants. Annals of Otology, Rhinology and Laryngology, Supplement 27, Vol. 85, May/June 1976, No. 3, Part 2), as did Hochmair-Desoyer IJ et al. (Four Years of Experience with Cochlear Prostheses, 1981, Medical Progress through Technology, Springer-Verlag, Vol. 8, pp. 107-119). More recent developments are summarized in the Proceedings of the European Symposium held in Hanover in June 1996 (American Journal of Otology, November 1997 Supplement. Lippincott-Raven).
Commercial cochlear implants currently available rely on surgery (see FIG. 4a) which places electronic parts 17 in the bone behind the ear known as the mastoid region 18 by drilling a small aperture 19 from the air cells in the mastoid region into the posterior part of the middle ear between the ear drum and the facial nerve 20. This nerve shown in cross-section in FIG. 4a, supplies the muscles of the face. Through this cleft electrodes are inserted either through or adjacent to the round window 21of the cochlea 52. Using this approach, access is obtained into the scala tympani.
The prior art approach described above is rather lengthy and has other significant limitations. The surgery is generally required to be done under general anesthetic, and the surgeon must navigate around several sharp bends which hinders full insertion of the electrode array into the scala tympani. Also, this approach requires significant drilling of the mastoid bone which creates a degree of bleeding from the bone vessels and marrow and provides a large raw bone surface area open to infection during surgery. Moreover, since the mastoid air cell system does not develop until approximately two years of age, this surgery is not possible for newborns. The bony dissection is extensive and there is a risk of damage to the facial nerve. The small access into the cochlea through the gap between the facial nerve and the tympanic membrane makes insertion of the flexible, delicate electrode array fully into the scala tympani very difficult and imposes a curvature in the line of insertion (see FIG. 4a). The present invention provides an improved method for surgically implanting a cochlear implant which overcomes these limitations of the prior art approach.
In our co-pending application entitled Inner Ear Implant Device filed contemporaneously with this one, the disclosure of which is incorporated herein by reference, a cochlear implant is described which comprises two elongated electrode-bearing prongs allowing insertion of one prong into the scala tympani and the other prong into the scala vestibuli for improved hearing percepts by the patient. It is an object of the present invention to provide a method for surgically implanting a cochlear implant as described in our co-pending application. It can also be adapted for use with conventional electrode arrays.
Surprisingly, the dimensions of the cochlea are remarkably constant from infancy to adulthood. Numerous anatomical studies of the scalae have been made, for example, see Takagi A, Sando I., Computer-aided Three-dimensional Reconstruction: A method of Measuring Temporal Bone Structures Including the Length of the Cochlea, Annals of Otology Rhinology and Laryngology, 1989, 98:515-522. These dimensions are critical in the design of any tools or methods to implant optimum performing stimulation devices into the cochlea of deaf persons. It is a further object of this invention to take advantage of the consistency in the dimensions of the cochlea by providing a template for use in assisting the surgeon in implanting a cochlear implant using the method of the invention.
It is yet a further object of the invention to provide a tool for keeping the surgeon's field of view unobstructed during surgery according to the method of the invention.
Current implant data suggest that proximity of electrodes located near the inner wall of the cochlea, where the nerves gather into the central "core" of the cochlea (modiolus), as well as the density of electrodes, have a positive effect on the performance of the implantee's speech percepts. Since present electrode arrays tend to position themselves naturally along the outside wall of the scala tympani during surgical implantation, it is difficult to stimulate discrete areas where nerve cells may still be functional. Thus, the consensus appears to be that it is better to position the electrodes as close as possible to the modiolus, as evidenced by the many designs that have attempted to orient the electrode array to get closer proximity to the modiolus. For example, Hansen et al. in U.S. Pat. No. 4,284,085 describes an implant design that uses "tabs" which contact the inner wall of the cochlea to enhance both the positioning and the proximity of the electrodes to the auditory nerves. Byers et al. in U.S. Pat. No. 4,686,765 details a method for manufacturing a pre-shaped electrode that preferentially curls to the inner wall of the cochlea. Kuzma further describes a method for altering the shape of an electrode in situ through the use of bio-absorbing materials to engage the inside turn of the cochlea in U.S. Pat. No. 5,578,084. Parker et al. details the use of bio-resorbable materials to allow an electrode to change shape after insertion in the cochlea in U.S. Pat. No. 5,653,742. In U. S. Pat. No. 4,261,372 Hansen teaches a multiple prong electrode design for addressing different turns within the same scala of the cochlea to achieve maximum insertion distance. It is therefore a further object of the invention to provide a means for ensuring that the electrodes of a cochlear implant are positioned as close as possible to the walls of the scalae.
The foregoing and other objects of the invention will be discerned from the summary of the invention and the detailed description of the preferred and alternative embodiments which follow.